I left out the following sentences. Thank you very much for your invaluable help, Writeacher.
1) On my summer holiday(s) I’m competing in a chess tournament in the first week of July. It will probably be difficult because the best players will participate.
2) Where will you go for your summer holidays? I’m looking forward to hearing your news. (or hearing from you?)
3) In the first week of my holiday I won’t do any homework because I’m tired.
You must listen to your teachers’ explanations ….. and a lot of others (better: there are a lot of other rules).
4) I’m going on a scout camp for ten days.Sorry, I’ve been out of touch but I’ve had a busy month.
Write to me soon. It’s all my news for now. Take care.
5) I mustn’t copy mu schoolmates’ work.
I have to/must enter the school at ten to eight. We should bring some money to school to buy something too eat and drink.
6) Your parents must justify your absences on the school slip.
You mustn’t modify the marks on your report card. The school year is going to finish very soon.
7) Our parents must fill in an excuse slip if we were ill. We have to spend the break outside the class.
8) I will be in Spain next week. I hope I will enjoy it a lot but I should return very soon.